One of the most widely produced chemicals worldwide is ammonia, which has applications as a fertilizer, a hydrogen storage media, and as a reactant in selective catalytic reduction of combustion gases from vehicles and stationary facilities, amongst many others.
The Haber (or Haber-Bosch) process is the principle manufacturing method for synthesizing ammonia. In the Haber process, ammonia is synthesized from nitrogen and hydrogen gas according to the following reaction:N3+3H2→2NH3  Equation (1)The Haber process employs an iron-based catalyst and operates at high temperatures (e.g., above about 430° C. (about 806° F.)) and high pressures (e.g., above about 150 atmospheres (about 2,200 pounds per square inch)), which lead to high-energy consumption. In addition, the ammonia conversions are relatively low, e.g., between about 10% and about 15%.
Due to these extreme process limitations, several researchers have investigated the synthesis of ammonia through an electrochemical approach. However, thus far, all the electrochemical routes presented in the literature had been performed in the solid state, which implies the use of solid and/or composite electrolytes. Therefore, the transport of the ions is limited by temperature. The electrochemical reactions reported in the literature are based on the transport of protons in which the reduction of nitrogen takes place according to:N2+6H++6e−→2NH3  Equation (2)while the oxidation of hydrogen takes place according to:3H2→6H++6e−  Equation (3)
Operating temperatures in the different systems that have been described in the literature range from 480° C. to 650° C., using perovskite-type, pyrochlore-type, and fluorite-type solid-state proton conductors as electrolytes. In addition to the high operating temperatures, the ammonia formation rates are low, with the highest reported rate in the order of 10−5 mol/s m2. Lower temperatures have been achieved with the use of Nafion®-type membranes allowing ammonia formation rates in the order of 1×10−4 mol/s m2 at 80° C. to 90° C. However, the operating voltages for the cell are high, in the order of 2.0 V, which represents a high energy consumption for the synthesis.
In view of the foregoing, there is a need for new methods for synthesizing ammonia.